I. Field of the Invention
This invention relates to signal processing and rate modulation in automatic cardiac pacers and, more particularly, to a rate adaptive pacer which provides pacing signals to the heart based on a root mean square approximation of band limited, low frequency, low level acceleration signals generated from body motion.
II. Description of the Prior Art
Activity sensing rate-responsive pacemakers have been developed which use body vibration during physical activity as an indicator of the need to alter pacing rate. One such prior art device is described in U.S. Pat. No. 4,428,378 to Anderson, et al., entitled "Rate Adaptive Pacer". The Anderson, et al. pacemaker operates by responding to a variety of mechanical forces both internal and external to a pacemaker patient. Because the Anderson, et al. pacemaker responds to forces applied to the patient's body, it must necessarily rely on the outside environment to provide a mass to generate the measured forces. Such masses as provided by the outside environment are subject to wide variations. Yet another deficiency of such prior art sensors is that they can measure only changes in pressure, not constant pressure. The present invention provides an activity sensing rate-responsive pacemaker including an accelerometer which contains a captive, well defined, constant reference mass.
Prior art rate adaptive pacemakers which are dependent on mechanical force sensors have been found to be relatively insensitive to different levels of exertion, particularly with respect to providing appropriate rate changes in response to a patient ascending stairs and walking up inclines. Such pacemakers are also susceptible to the effects of extraneous vibration such as that encountered in various forms of transport. One such study is reported by C.P. Lau, et al. in an article entitled "Selective Vibration Sensing: A New Concept for Activity--Sensing Rate-Responsive Pacing," P. 1299, PACE. Vol. II, Sep. 1988.